1. Field of the Invention
The present invention relates to an acoustic wave filter device including resonators, a radio-frequency front-end circuit, and a communication apparatus.
2. Description of the Related Art
An existing acoustic wave filter device, such as a ladder filter, is configured such that a single parallel arm (parallel-arm resonant circuit) connecting a series arm (series-arm resonant circuit) and ground is provided with a plurality of parallel-arm resonators (see, for example, Japanese Unexamined Patent Application Publication No. 2009-207116). In this configuration, a plurality of parallel-arm resonators having different resonant frequencies are disposed in a single parallel arm so as to be connected in series with each other, to achieve an improvement in the skirt characteristics (attenuation slopes) of a filter.
However, when the existing acoustic wave filter device described above is defined by resonators that use acoustic waves, a parallel-arm resonator having a low resonant frequency and a low anti-resonant frequency among a plurality of parallel-arm resonators disposed in the same parallel arm may generate ripples which result in an increase in loss within the pass band.
Specifically, a resonator that uses an acoustic wave is defined by an IDT (InterDigital Transducer) electrode and reflectors having a periodic structure including a periodic array of conductive strips (electrode fingers). With this configuration, such a resonator has a frequency band over which acoustic waves in a specific frequency region are reflected with high reflection coefficients. Such a frequency band is generally referred to as a stop band and is defined by the repetition period of the periodic structure. At this time, ripples that are local fluctuations in reflection coefficient are likely to occur at the high-frequency end of the stop band.
In the existing acoustic wave filter device described above, the high-frequency end of the stop band of a low-frequency parallel-arm resonator may be located in the pass band or near the high-frequency end of the pass band. In this case, therefore, ripples occurring at the high-frequency end of the stop band of this parallel-arm resonator may increase the loss within the pass band.